Yes, it's a no-no ...

It is an exploit of a poor map design coupled with a bad spawn system.

This is totally possible to do on some maps (mission and terminal come to mind) where you can be very close to the spawn and it doesn't flip.

It is very poor sportsmanship... but seriously, if someone spawns right in front of you... are you not going to shoot?  I appreaciate they should not be spawning there... but seriously?

In DOM it's hit or miss.  I'd say if they are strategically positioned to where you are spawning in front of them and completely unable to move, that is definitely bannable.  If that's not the case, then they won't be punished.  Is it fair to the enemy team?  No.  But the devs aren't concerned with spawn trapping, it's the spawn killing that they are releasing bans for. 

I've personally only experienced spawn killing twice in my MW3 career.  Both times was on DOME with a certain clan I won't mention here.  Each enemy was positioned throughout the map to where when you spawned, you either spawned right in front of them, or very near their position so that they didn't even have to move to get kills.  They just sat there in each of their corners, popping people off as they re-spawned.  That is a bannable offense.  In DOM it is VERY hard to say whether a team is spawn killing or just trapping you at a certain point.  Funny thing is they thought they were good for doing said spawn killing.  So very sad this world we have to live in. 

Cut and paste this thread to the BO2 forum, it's going to be exactly the same, apparently.

First of all, taking advantage of choke points on the battlefield is tactical. Abusing game mechanics (someone i know sets up with his friends in hardcore and basically shootd through the wall at the enemy teams spawn point and repeatedly get MOABs). Anything like that makes you a 'try-hard'. Yes you achieve it but not honestly or fairly.

In domination, two flags are all that are needed. By running into the third, you only make the enemy spawn elsewhere which can catch your team mates by suprise. Yes its more fun to continually traverse the map but if you want to win, take two flags and guard the routes leading from the enemy's territory.

I talk about that in my combat guide entitled FPS Academy (on amazon).

Extreme spawn abuse is just douchy and lame. If you can't get a MOAB or a win by yourself, you don't deserve one by using abusive tactics with your team.